1. Field of the Disclosure
The present invention relates to a double-disc gate valve, the shut-off discs of which are displaceable from an open position into a closed position and vice versa by means of an actuator rod, there being associated with the actuator rod a locking device by means of which the valve discs, together with the actuator rod, are fixable in the closed position.
2. Description of Related Art
In respect of the construction of double-disc gate valves, reference is made to EP 1 561 980 A1, EP 0 450 646 A2 or WO 98/17934. The respective constructions all originate from the Applicant and its predecessors.
Those documents do not disclose a locking device by means of which the valve discs, together with the actuator rod, are fixable in the closed position, but such locking devices are also part of the prior art. They are distinguished by a locking wedge which is displaceable perpendicularly to the actuator rod. The actuator rod is constructed so as to be complementary thereto. The disadvantage of such a construction is that locking can take place only in a rigidly defined position of the actuator rod. It should be borne in mind, however, that the valve discs are subject to wear. It should also be borne in mind that the resilient bias in the region of the sealing seat between the valve discs and the valve housing varies in the course of time. Accordingly, the so-called “compression distance” varies from stroke to stroke, that variation being in the range of from 12 mm+/−8 mm, in order to generate the necessary sealing force between the valve discs and the housing sealing seats. With rigidly positioned locking of the actuator rod for the valve discs, the valve discs therefore cannot be brought into exactly the optimum closed position having the maximum sealing force. Consideration must also be given to the fact that the actuator rod, once the closed position has been reached, moves back again by a certain amount of up to 2 to 3 mm, that is to say until the coupling piece arranged on the actuator rod is in contact with the locking wedge. That behaviour involves the risk that the valve will not be sealed in the closed position.
Because the locking wedge is mounted so as to be movable back and forth perpendicular to the actuator rod and to the drive column associated with the actuator rod, it is also necessary for the drive column to be extended accordingly, consequently requiring a corresponding amount of space.
In the event of an operational malfunction, the full opening force of the hydraulic drive means, plus the spring force of the cup springs in the valve, has be taken by the locking wedge and the corresponding counter-bearings on the drive column and actuator rod. This results in an adverse flow of force and consequently in correspondingly high expenditure in terms of materials. The locking wedge and associated counter-bearings must be of correspondingly strong construction. The same is true of the drive column inside which the actuator rod is arranged so as to be movable back and forth.
It must therefore be ensured that the valve, i.e. the valve discs, remain in the compressed locking position in order to ensure a sealed and reliable closure of the valve. The high contact pressure must be ensured even after the drive means for the valve discs or the actuator rod thereof have been deactivated. In detail:    1. the restoring force of a cup spring assembly acting in the opening direction (about 700,000 N), which force is related to the nature of the construction in accordance with the double-disc principle, must be countered;    2. potential opening of the valve initiated by faulty operation of the hydraulic drive means (about 1,800,000 N) must be fully countered;    3. the compression distance, which varies as a result of the principle of the force-controlled compression and as a result of heat-related expansion, must be fully compensated by the locking means (about 12 mm+/−8 mm).
Furthermore, it must be ensured that the locking can be fully cancelled prior to a fresh opening stroke.